Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery or examination. A tracking or navigation system may provide positioning information of a medical instrument (such as a drill, a catheter, scalpel, scope, stent or other tools) with respect to the patient or a reference coordinate system. A medical practitioner may refer to the tracking system to ascertain the position of the medical instrument when the instrument is not within the practitioner's line of sight. A tracking system may also aid in pre-surgical planning.
The tracking (or navigation) system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation (herein, “P&O”) of the medical instrument. The medical practitioner may use the tracking system to determine when the instrument is positioned in a desired location. The medical practitioner may locate and operate on a desired or injured area while avoiding other anatomical structures.
Tracking systems may be ultrasound, inertial position, or electromagnetic tracking systems, for example. Known electromagnetic tracking systems employ coils as receivers and transmitters. The focus herein is on electromagnetic tracking systems (herein, “EM trackers”). In EM trackers, transmitter coil or coils emit quasi-static magnetic fields and receiver coil or coils measure the fields as received. From the field measurements and mathematical models of the coils, the P&O of the receiver with respect to the transmitter is determined. Alternatively, the P&O of the transmitter with respect to the receiver is determined. From this, the P&O of the medical instrument is determined with respect to the relevant anatomy of the patient.
EM trackers can be built with various coil architectures. Industry-standard-coil-architecture (ISCA) EM trackers use a trio of nearly-co-located nearly-orthogonal nearly-dipole coils for the transmitter and another trio of nearly-co-located nearly-orthogonal nearly-dipole coils for the receiver. Each coil trio is carefully characterized during manufacture to numerically express the precise value of the “nearly-” attribute in the previous sentence. From the field measurements and mathematical models of the coils, the P&O of the receiver with respect to the transmitter is determined. Alternatively, the P&O of the transmitter with respect to the receiver is determined. All six degrees of freedom (three of position and three of orientation) are tracked.
Single-coil EM trackers use a single dipole or nearly-dipole transmitter coil and an array of six or more receiver coils, or else use a single dipole or nearly-dipole receiver coil and an array of six or more transmitter coils. By electromagnetic reciprocity, these two arrangements function equivalently. The coils in the array may be dipole, nearly-dipole, or non-dipole coils (or combinations). The coils in the array are either precisely manufactured or precisely characterized during manufacture to obtain mathematical models of the coils in the array. The single coil does not need to be characterized.
From the field measurements and mathematical models, the P&O of the single coil with respect to the array are tracked. Since the single coil is symmetrical about its roll axis, only five degrees of freedom (three of position and two of orientation) of P&O are tracked. The gain of the single coil is also tracked.
The array of coils can be fabricated as a printed-circuit board or as an array of wound coils or as a combination of both. Arrangements of coils in the array vary widely in various implementations of single-coil EM trackers. The array may include electrically-conductive or ferromagnetic materials as part of the design of the array.
Typically, a tracker receiver unit (of course, this could be a transmitter unit) is attached to the to-be-tracked medical instrument. In addition, a tracker receiver unit is attached to other components that are also tracked. For example, with a conventional fluoroscopic image-guided procedure, one or more ISCA receivers are mounted on an X-ray image detector of the fluoroscope. When taking a fluoroscopic image, the tracking system tracks the X-ray image detector via the ISCA receivers mounted thereto. In this way, the tracker can determine the P&O of the X-ray image detector with respect to the relevant anatomy of the patient and, thus, be able to determine the relative P&O of other tracked medical instruments and components.
In addition to conventional fluoroscopic image-guided procedures, a tracker receiver unit is also attached to a surgical microscope for surgery, for example, inside the skull. In this exemplary application, an ISCA transmitter is rigidly fixed to the patient's skull to provide the dynamic reference to the patient's anatomy. One or more ISCA receivers are attached to the surgical instrument to track the P&O of the instrument with respect to the ISCA transmitter, and thus with respect to the patient's anatomy.
The real-time position and orientation of the instrument are superimposed on pre-operative images of the patient's anatomy. One or more ISCA receivers are mounted on the surgical microscope to permit tracking the microscope's line-of-sight with respect to the ISCA transmitter, and thus with respect to the patient's anatomy. The position of the microscope's focal point along the microscope's line-of-sight is read from the microscope. This information permits the position of the microscope's focal point to be determined with respect to the ISCA transmitter, and thus with respect to the patient's anatomy. The real-time P&O of the microscope's focal point and focal axis are then superimposed on pre-operative images of the patient's anatomy. The real-time P&O of the instrument are also superimposed on pre-operative images of the patient's anatomy.
In these exemplary contexts (conventional fluoroscopic image-guided procedures and microscope image-guided procedures) and other similar contexts, one major difficulty is the electrically-conductive materials and ferromagnetic materials (herein, “distorters”) in the medical components distort the magnetic fields near the ISCA receivers mounted to those components. In the case of conventional fluoroscopic image-guided procedures, the distorters in the X-ray image detector distort the magnetic fields near the ISCA receivers mounted to the detector. In the case of conventional microscope image-guided procedures, the distorters in the microscope distort the magnetic fields near the ISCA receivers mounted to the microscope. ISCA EM trackers are very sensitive to such field distortion, leading to inaccurate tracking or, in extreme cases, a failure to track at all.
Two approaches are conventionally employed to ameliorate the distortion caused by the distorters in an image-capture component (such as an X-ray image detector or a surgical microscope): Distortion-mapping and calibration and spaced-mounting.
Distortion-mapping and calibration: The distortion caused by a detector's distorters is measured and mapped during a manufacturing-time-intensive robotic mapping procedure. In this way, the tracking errors are mapped and calibrated during use. Also, the limitations of accurate tracking can be determined before actual use. In other words, the accurate P&O of the detector might not be determinable under defined conditions.
Spaced-mounting. The ISCA receiver is rigidly mounted in a manner so as to put distance between the distorters of the detectors and the receiver itself. In this way, the receiver is spaced away from the surface of the detector to reduce the effects of field distortion. This conventional approach is illustrated in FIG. 1. An image-capture component 100 is generically shown as a box. Examples of an image-capture component include an x-ray image detector or a surgical microscope. A mounting-bracket 110 attaches an EM receiver 120 to the image-capture component 100. The mounting-bracket 110 is designed so that the EM receiver 120 is physically spaced away from the surface of the image-capture component 100 (and its distorters therein).
Consequently, in the conventional approach, the following occurs in an effort to reduce the effects of distortion caused by the distorters in image-capture components:                The image-capture component cannot be tracked in some desired positions and orientations.        The receivers stick out from the image-capture component, so get in the way.        An expensive mapping manufacturing process is necessary.        